Honestly,
From experience & from talking to people, scripting is always the best way to go, whether it's for the intro, a review of a game, it's nice to put things down in writing so that your videos aren't all over the place & they're organised! At least that way you have a general idea of what you're saying, when's a good time to say them etc. Obviously don't make it overwhelming scripted where people can tell & have a little bit of improv, just relax, imagine it's just you playing the game!
Once you get comfortable, you can slowly stop scripting certain parts of the video & then go with the flo!